Known conventional technologies of applying a mask to a pyroelectric sensor include Patent Literature 1. Patent Literature 1 sets or adjusts the sensing area of a pyroelectric sensor by applying a mask to the lens of the pyroelectric sensor.
A pyroelectric sensor uses the pyroelectric effect of an element called a pyroelectric material (hereafter referred to as a “pyroelectric element”), such as a ferroelectric ceramic. A pyroelectric effect refers to a phenomenon in which thermal energy based on a small amount of infrared rays emitted by an animal, human body, or the like (hereafter simply referred to as a “human body or the like”) causes a temperature change, which then induces electrical charge on the surface of the pyroelectric material, thereby generating an electromotive force. A human body or the like emitting infrared rays is hereafter referred to as a moving object.
The operation principle of a pyroelectric element 91 will be described with reference to FIGS. 1 and 2. As shown in FIG. 1, the pyroelectric element 91, which has been subjected to polarization treatment, is stably polarized at a given temperature T [° C.]. In this state, positive float charge and negative float charge are adsorbed on the negatively polarized surface and positively polarized surface, respectively, due to the nature of static electricity, and there is no potential difference between the upper and lower surfaces of the pyroelectric element 91.
A blackened film (not shown) is disposed on the infrared entry side of the pyroelectric element 91. When infrared rays enter the blackened film, the blackened film converts the infrared energy into thermal energy, thereby changing the temperature of the pyroelectric element 91 by ΔT [° C.]. Since the polarization of the pyroelectric element 91 depends on the temperature, the magnitude of the polarization in the pyroelectric element 91 varies with the temperature change, as shown in FIG. 2. At this time, the surface charge based on the floating charge cannot respond to the temperature change as quickly as to the polarization change. Accordingly, an amount of charge corresponding to the polarization change temporarily remains on the element surface. This charge generates an electromotive force, which in turn causes a current flow.
Next, a dual pyroelectric sensor 90 will be described. FIG. 3 shows a circuit diagram of the dual pyroelectric sensor 90. The dual pyroelectric sensor 90 has two pyroelectric elements, 91R and 91L, having different polarities disposed on the sensing surface thereof. The pyroelectric elements 91R and 91L are connected in series in reverse polarity. The pyroelectric sensor 90 outputs the difference between the outputs of the pyroelectric elements 91R and 91L. The dual pyroelectric sensor 90 thus configured has the following characteristics. (1) When a moving object moves in a direction which crosses the pyroelectric elements 91R and 91L (disposition direction), the voltage sequentially changes in the positive and negative directions, like that of an alternating current. Accordingly, a high output voltage can be obtained from the sensor circuit. (2) When external light such as sunlight simultaneously enters the pyroelectric elements 91R and 91L, the outputs thereof cancel out each other since the pyroelectric elements are connected in reverse polarity. Accordingly, the dual pyroelectric sensor 90 produces no output. As a result, a malfunction of the dual pyroelectric sensor 90 can be prevented. (3) Additionally, the dual pyroelectric sensor 90 is resistant to changes in the ambient environment such as vibration or temperature.
The dual pyroelectric sensor 90 having such characteristics is typically used in combination with a light-harvesting Fresnel lens. On the other hand, in order to detect a moving object at a relatively short distance, there have been commercialized pyroelectric sensors which are combined with a mask having an aperture pattern, such as a punching metal, in place of a Fresnel lens and thus are miniaturized.
FIG. 4 is an external view of a mask 93 attached to a lower portion of a liquid crystal display. FIG. 5 is a schematic diagram of the aperture pattern of the mask 93. FIG. 6 shows changes in a dead zone 82 (the shadow of the mask) when a moving object 81 crosses a pyroelectric sensor 80. FIG. 7A shows the positional relationship between the dead zone 82 and the pyroelectric elements 91R and 91L when the moving object 81 is located in front of the pyroelectric sensor 80. FIG. 7B shows the positional relationship between the dead zone 82 and the pyroelectric elements 91R and 91L when the moving object 81 moves in an x-direction (left direction) with respect to the pyroelectric sensor 80. FIG. 7C shows images of outputs when the dead zone 82 changes from that in FIG. 7A to that in FIG. 7B. Note that in FIGS. 6, 7A, and 7B, the aperture pattern and irradiated portion are shown not as a circular portion but as a region including the aperture pattern and irradiated portion for simplification.
It is known that when the dead zone 82, to which no infrared rays are applied, is arbitrarily generated on the pyroelectric elements 91R and 91L using a mask as described above, the pyroelectric sensor 80 can increase the sensitivity with which it detects a movement of the moving object 81. Assuming that the total area of each of the pyroelectric elements 91R and 91L is 90% for convenience, in FIG. 7B, the irradiated range of the pyroelectric element 91R increases by 30%; the irradiated range of the pyroelectric element 91L decreases by 30%. Accordingly, there is a difference of 60% therebetween. Use of this difference allows for increase of the detection sensitivity.